The use of electrophotographic printing (commonly known as laser and LED printing) within digital copiers and multifunction printers poses algorithmic challenges for copy mode image processing.
For many electrophotographic print engines, the highest overall print reproduction quality is obtained using halftoning based on line or dot screens. Such halftoning techniques may modulate image data with fixed 75-250 cycles/inch patterns prior to printing. In dark image areas, the duty cycle for toner application may approach 100%. In light areas, the duty cycle may approach 0%. The human internal perception system averages away the halftone pattern, leaving the illusion of smooth shades. However, printing using screen based halftoning techniques may produce print artifacts, such as when the frequency content of the image to be printed causes interference or “beat” patterns with the halftone screen modulation. Copy operations (as opposed to printing directly from a digital image) are particularly prone to interference artifacts as the source document often contains screen based halftoning itself, with frequency content similar to that of the print engine screening.
Interference patterns caused by overlapping patterns between image data and halftone screening may be reduced by applying a low pass filter to the image data prior to screening and printing the image. However, indiscriminately applying the low pass filter to the image data may yield unacceptable blurring of certain image features such as text.
Conventional solutions attempt to resolve this problem by limiting the application of low pass filtering to regions of the scanned document determined to be halftoned. These conventional solutions may yield acceptable results in some cases, but artifacts may still be observed when strong edges are present in halftoned regions. Examples include strong edges in halftoned photographic content regions and black text on a halftoned background. In each case, the lack of low pass filtering very close to the strong edges in combination with the application of low pass filtering in areas farther away from strong edges results in distracting interference patterns.
Another problem that electrophotographic print engines often face is misalignment of color constituents in color printing. Color printers based on electrophotographic techniques typically do not have perfect alignment of the constituent cyan (C), magenta (M), yellow (Y), and black (K) toner planes. Misalignments of color constituents vary over the width and height of the page, and could be up to 1/300 inches in conventional systems. For copy operations, reproduction of dark text is particularly prone to misalignment artifacts, as dark shades are typically produced by a mix of CMYK toners. Misalignment results in non co-located toner edges and thus, fuzzy or haloed text reproduction.
To compensate for electrophotographic toner misalignment during print (rather than copy) operations, most approaches rely on understanding the elements to be printed at the object level rather than the pixel level. In reproducing dark text, for instance, the print rendering will often generate C, M, and Y renderings of characters which are “eroded” from the full K rendering. As such, the full K rendering will eclipse any C, M, or Y which would otherwise sneak out due to misalignment. Yet, at the center of the text, the mix of C, M, Y, and K would still yield the desired color tone.
Compensating for misalignment during copy operations is much more difficult when an understanding of the image elements is not readily available. In the past, attempts have been made to compensate for electrophotographic toner misalignment during copy operations using a couple of different algorithms. For example, under one approach, regions of black text on white background are detected. The detected area is then reproduced with a separate K only color table. However, this approach is prone to have issues due to imperfect black text classification. In addition, there was no compensation for text which was dark yet not deemed black by the classification process.
Under another approach, the image processing algorithm is tuned to focus on edge sharpening and enhancement in the luminance component. Once processed, the edges of dark text in the image would tend to saturate on the neutral axis, near white and black. This processing helps to limit color (color other than black) at the edge of the text. A drawback of this approach is that it may require the printer to be configured such that color tones on the neutral axis are reproduced with K only.
In view of the foregoing, there is a continued need in the art for improvements in optimizing and processing images for printing to minimize artifacts introduced during the scanning and/or printing operations.